This invention relates to rebroadcast transceivers which are designed to receive RF (radio frequency) signals, amplify them, and retransmit them onward on the same frequency. Such transceivers are known in the broadcasting field as on-channel repeaters, and are sometimes termed active deflectors.
In an on-channel repeater, due to unwanted coupling or feedback between the receiving and the transmitting antenna, the repeater can also receive its own retransmitted output, thus causing instability and relaxation oscillations. Our International Patent Application WO97/16942 and European Patent Application 772310 (equivalent GB 2306082) describe a method and apparatus which has been found to be surprisingly effective in removing this feedback. In this method there is an amplification path between the input and output antennas which provides substantially linear processing and includes a delay sufficient to decorrelate the output and input. The repeater includes an amplification path providing substantially linear processing without demodulation and decoding, and a filter-estimator responsive to the signal in the amplification path for correlating the signal in the amplification path before the delay with a noise-like signal taken after the delay to produce a plurality of correlation coefficients. The filter estimator may use the least mean square method. An adaptive filter in the form of a transversal filter receives the signal in the amplification path and is controlled by the control coefficients to provide a modified signal, and a combiner combines the modified signal with the signal in the amplification path so as to reduce the effect of the feedback. In this way, unwanted feedback from the output of the active deflector to the input is substantially eliminated. The compensation conveniently makes use of the inherent noise-like property of the signal, as described it is an OFDM signal; however a separate noise signal may be added if necessary.
An example of a transceiver of the type described in our earlier applications is shown in FIG. 1. The transceiver 10 has a receiving antenna 12 which is coupled to an amplification path 14 which includes an adder 16, a decorrelating delay 18, and an amplifier 20. The output of the amplifier is applied to a transmitting antenna 22. The output of the delay 18 is also applied as a reference signal to a filter estimator 24, which also receives the output of adder 16, and an adaptive filter 26, which applies an output to the subtractive or inverting input of adder 16. The construction and operation of the whole of this corrector circuitry 28 is described in detail in our earlier applications. Part of the output of the transmitter antenna will be picked up by the receiving antenna as indicated by the dashed line 30, as unwanted feedback. The corrector 28 removes the effect of this feedback.
An on-channel repeater can be used for two purposes, namely coverage extension and hole filling. Coverage extension is used if the received signal strength is insufficient in a particular area, and the repeater is then used as an additional transmitter, often as a part of a Single Frequency Network (SFN).
In hole-filling, the received signal strength is sufficient, but signals on adjacent channels to the desired signal are so strong that domestic receivers have insufficient dynamic range to demodulate the wanted channel successfully. In extreme cases, in areas very close to adjacent channel transmitters, the intermodulation products from the adjacent channels interfere directly with the wanted signal, even though spectral mask requirements have been fulfilled. The repeater must remove the intermodulation products of co-sited adjacent channels that arrive at its input, as well as removing the unwanted feedback. Referring to FIG. 1, we have appreciated that any interfering signal, illustrated at 32, generated externally to the transceiver may be combined with the transmitter signal as illustrated by the notional combiner 34 and contribute to the unwanted feedback.
FIG. 4 illustrates possible requirements on such a repeater. The figure is a spectral diagram showing the signals in and adjacent a wanted channel at 225.648 MHz when there is a strong signal on the adjacent channel at 227.360 MHz. Typically, the adjacent channel signals can be up to 53 dB larger at the input than the received wanted signal, imposing stringent requirements of selectivity of filters, linearity of mixers and dynamic range of digital-to-analog converters. At its main input, fed from the receiving antenna, the device should cope with a parasitic feedback-to-received signal ratio of up to 35 dB, while the cancellation of this feedback should be at 45 to 50 dB in order not to degrade the signal-to-noise ratio of the recovered signal. According to standard EN50248:2001, Characteristics of DAB Receivers, Section 7.3.3, a consumer DAB receiver must be able to decode a signal surrounded by adjacent channels at +30 dB and cope with any signals 5 MHz away from the centre frequency at +40 dB.
In the known system of our earlier applications referred to above, the input of the adaptive filter is connected internally to the output of the repeater device as described above and shown in FIG. 1. We have appreciated that because this clean internal reference might differ from the actual signal transmitted through the antenna, the signal recovered in the cancellation stage will still contain any products and interferers which are uncorrelated with the reference.